allow to listen on Unix socket

[ibukanov]

When the real web server serves as a proxy for the tool, redirecting the traffic for /.well-known/acme-challenge using unix sockets allows for more resilient/hardened setup. For example, one can use permissions or selected mounts to isolate tool's socket from the rest of the system. So it would be nice if -s option to the cert command allowed to specify a unix socket to listen.